The present invention relates, in general, to integrated circuits and, more particularly, to integrated circuits having an image reject transceiver.
In electronic systems such as cellular or wireless telephones, and image detectors for televisions, among others, a phase and frequency relationship exists between an incoming reference signal and a signal generated by a Voltage Controlled Oscillator (VCO). Typically, mixers in a heterodyne receiver translate a Radio Frequency (RF) signal to a lower Intermediate Frequency (IF) signal. The mixers multiply the received RF signal with a Local Oscillator (LO) signal to generate the sum and difference frequencies between the RF and LO signals. A filter circuit is used to select the difference frequency, i.e., a mixed down signal, for recovering the modulation information that is contained in the translated RF signal.
Unwanted images having frequencies either higher or lower than the LO signal are also present at the input of the receiver. The unwanted images are also translated in frequency by the mixers. It is desired that the signals at the outputs of the mixers be in-phase while the unwanted images be one hundred and eighty degrees out-of-phase and equal in amplitude. By summing the signals at the outputs of the mixers, the unwanted images being out-of-phase and equal in amplitude would completely cancel, while the desired signals would add.
However, it is difficult to maintain equivalent signal attenuation and a phase relationship of one hundred and eighty degrees between the unwanted images with voltage and temperature changes in a heterodyne receiver. Further, a typical heterodyne receiver includes two passive filters that provide rejection of the unwanted images. Requiring filters for the input RF signal to reduce the amplitude of the unwanted image adds additional costs to the heterodyne receiver.
Accordingly, it would be advantageous to have a method and circuit for frequency translating an incoming reference signal that causes unwanted images to be rejected. It would be of further advantage to provide both gain attenuation and a phase relationship that dynamically correct the translated signal for complete image rejection.